A coding method is proposed which combines a CELP (Code Excited Linear Prediction) coding method suitable for a speech signal with a transform coding method suitable for a music signal in a layer structure, as a coding method which can compress speech and music and so forth at a low bit rate and with high sound quality (see for example, Non-Patent Literature 1). Hereinafter, a speech signal and a music signal are collectively referred to as an audio signal.
In the coding method, a coding apparatus first encodes an input signal by a CELP coding method to generate CELP coded data. The coding apparatus then converts a residual signal (hereinafter, referred to as a CELP residual signal) between the input signal and a CELP decoded signal (a decoded result of the CELP coded data) into the frequency domain to acquire a residual spectrum and performs transform coding on the residual spectrum, thereby providing a high sound quality. A transform coding method is proposed which generates pulses at frequencies having a high residual spectrum energy and encodes information of the pulses (see, Non-Patent Literature 1).
While the CELP coding method is suitable for speech signal coding, the coding model of the CELP coding method is different from that of a music signal, and therefore sound quality degrades in coding the music signal through the CELP coding method. For this reason, the CELP residual signal component is large when the music signal is encoded by the above coding method, and thereby raising a problem that sound quality is less likely to be improved in encoding the CELP residual signal (residual spectrum) by the transform coding.
To solve this problem, a coding method (a CELP component suppressing method) is proposed which suppresses the amplitude of a frequency component of the CELP decoded signal (hereinafter, referred to as a CELP component) to calculate a residual spectrum and performs transform coding on the calculated residual spectrum to provide high sound quality (see, for example, Patent Literature 1 and Non-Patent Literature 1 (section 6.11.6.2)).
The CELP component suppressing method disclosed in Non-Patent Literature 1 suppresses the amplitude of the CELP component (hereinafter, referred to as CELP suppressing) in only a middle band of 0.8 kHz to 5.5 kHz when a sampling frequency for an input signal is 16 kHz. In Non-Patent Literature 1, the coding apparatus does not directly perform transform coding on the CELP residual signal, and reduces the residual signal of a CELP component by another transform coding method beforehand (see, for example, Non-Patent Literature 1 (Section 6.11.6.1)). For this reason, the coding apparatus does not perform CELP suppressing on a frequency component coded by the other transform coding method even in the middle band. A CELP suppressing coefficient indicating the degree of CELP suppressing (level) is constant in frequencies in the middle band other than frequencies in which the CELP suppressing is not performed. The CELP suppressing coefficients are stored in a code book (hereinafter, referred to as a CELP component suppressing code book) according to the level of the CELP suppressing. The CELP component suppressing code book stores a coefficient (=1.0) meaning that no CELP component is suppressed.
The coding apparatus performs CELP suppressing by multiplying the CELP component (a CELP decoded signal) by the CELP suppressing coefficient stored in the CELP component suppressing code book before the transform coding, acquires the residual spectrum between the input signal and the CELP decoded signal (a CELP decoded signal after the CELP suppressing), and performs transform coding on the residual spectrum. The coding apparatus then calculates a residual signal between the input signal and a signal obtained by adding a decoded signal of the transform-coded data and the CELP decoded signal in which the CELP component is suppressed, searches for a CELP suppressing coefficient such that an energy of the residual signal (hereinafter, referred to as a coding distortion) is minimum by a closed loop, and encodes the searched CELP suppressing coefficient. By this means, the coding apparatus can perform transform coding which minimizes the coding distortion in all bands. Meanwhile, a decoding apparatus suppresses the CELP component of the CELP decoded signal using the CELP suppressing coefficient transmitted from the coding apparatus and adds a decoded signal subjected to transform coding to the CELP decoded signal in which the CELP component is suppressed. This allows the decoding apparatus to acquire a decoded signal having less deterioration of sound quality due to CELP coding when performing coding which combines the CELP coding and the transform coding in a layer structure.